It is well know to the art that the cleanliness of an electronic assembly is critical to the assembly's field performance and reliability. Unfortunately, corrosive levels of ionic and nonionic residues may build up during the fabrication and assembly processes, and may also arise from external sources during field service. These residues frequently cause electrical shorting or corrosion failures to occur through electromigration or electrical leakage between two circuits.
Since passage of the Clean Air Act in 1987, many new and creative fabrication and assembly processes have emerged and are successfully being used to build hardware. In these modern methods the traditional use of chlorinated solvent cleaners has largely been discontinued, with aqueous, semi-aqueous or new nonchlorinated solvent cleaners now being preferred.
Modern electronic assembly methods are having greater field performance problems, particularly with respect to the contamination referred to above. For example, the new assembly methods which use no rosin or small amounts (<5%) of rosin do not have the protective insulative layer that traditional methods provided. Thus, the new processes are having failures due not to the materials used during assembly, but due to the normal fabrication and handling residues, or because the cleaning was only designed to remove the majority of the flux residue and not the layer of corrosive fabrication residues below the flux or beneath the components. These are new issues for the electronic assembly industry, because it was previously believed that solvent cleaning removed all fabrication and assembly corrosive residues instead of sealing them in with a clear thin layer of rosin.
Although research laboratory analytical tools such as ion chromatography (IC) and high pressure liquid chromatography (HPLC) may be used to separate, identify and quantify the ionic and nonionic residues present on electronic circuit assemblies, these are not production floor process control tools. The current inventive process control tools were developed for monitoring rosin flux and solvent cleaning residues, and are not capable of measuring the true cleanliness of an electronic assembly. Instead, current process control tools are merely gross performance indicators (trend tools) of the processing equipment. With these trend tools, less than 30% of the residues come into solution during extraction, and generally a layer of flux residue, as well as fabrication residue, remains below this layer.
More particularly describing the prior art methods, most prior art trend tools use a solvent water (75% isopropyl alcohol and 25% water) extraction for 5-15 minutes at room temperature or at a slightly elevated temperature (limitation due to flammability), followed by measuring the total change in the conductivity of the solution over the time of the test. The resulting conductivity change is then compared to a conductivity salt standard (e.g., sodium chloride at about 750 ppm). There is no differentiation between corrosive and non-corrosive residues.
One further disadvantage of most prior art methods is that they are designed to extract whole- or half-board areas, and then to normalize the data to provide per unit area calculations (so much NaCl equivalents per square inch). Thus, those prior art methods are not effective for performing comparative cleanliness analysis (for corrosive residues) of various processing step effects, such as: (1) wave solder area vs. surface mount area; (2) top of board after wave solder vs. bottom of board; (3) bare incoming vs. assembled areas; (4) reworked areas vs. non-reworked areas, etc.
It can be seen from the above that a need exists for an improved, nondestructive spot extractor for IC and HPLC analysis to determine the actual contamination levels in specific areas on the board. The present invention addresses that need.